Gold may be produced hydrometallurgically using cyanide leaching or hydrochloric acid leaching. Gold ores, concentrates, oxidation residues and calcines are almost exclusively subjected to cyanide leaching with carbon in leach, carbon in pulp, or cyanide leaching followed by gold cementation using zinc (Merrill-Crowe process). Hydrochloric acid leaching with high acidity and high redox potential is mainly applied for secondary raw materials including high concentrations of platinum-group metals (PGM), such as anode slimes. At present, there is a negligible amount of industrial scale cyanide free gold processes for gold ores, concentrates, oxidation residues, calcines or low grade secondary raw materials.
Already in the 19th century, gold was leached by using chlorine gas. By using chlorine gas, the redox potential of a leach solution may be increased and gold dissolved from the raw material as a gold chloride complex. One of the major issues at that time was gold precipitation on production equipment materials, e.g. on the rotating wooden barrel. Sometimes at the end of the leaching plant/equipment lifetime, the barrels were burnt to recover gold. There were attempts to prevent that phenomena e.g. by tar on the wooden surface. This tendency (also referred to as preg-robbing, resorption, solution depletion) of the gold chloride complex to precipitate/reduce back to the raw material or other material present in the process has caused problems when developing chloride processes.
Development stage gold chloride leaching processes generally use high concentrations of chloride, typically with a bromide addition and an oxidant (such as Cu2+) present. This is in order to dissolve gold and to keep gold stable in the solution. It is generally accepted that decreased chloride concentration has a negative impact on gold leaching kinetics. However, high chloride and bromide concentrations may cause high operating costs such as increased amount of wash water or increased loss of chemicals to the bleed and/or leach residue and increasingly complex chemistry in the process.
For example AU2008202814 discloses a method for recovering gold and other precious metals using halide solutions.
AU201320205 discloses a method of eluating gold and silver comprising leaching gold and silver using acidic leachate which includes chloride ions and/or bromide ions as anions and copper and iron as cations from sulphide ore bearding cold and silver to the acidic leachate by heating; absorbing at least the gold and silver in the acidic leachate on activated carbon and eluting gold and silver on activated carbon on which at least the gold and silver are adsorbed by using an aqueous thiosulfate solution that is maintained at less than pH 7. The sequential method of AU201320205 does not appear to allow recovery of gold from preg-robbing raw materials.
One of the problems associated with the methods disclosed in AU2008202814 and AU201320205 is that high halide concentrations (e.g. chloride or bromide) are required resulting in requirement to clean all process waters bled from the process before they can be released to the environment and increase process chemical cost, industrial water and optionally evaporation cost.